Method for removing oxides of nitrogen from gaseous mixtures

ABSTRACT

A method for removing oxides of nitrogen from gaseous mixtures by contacting the gaseous mixtures with alkali manganate.

United States Patent Okabe et al.

[ Dec. 30, 1975 [73] Assignee: Mitsubishi Kinzoku Kabushiki Kaisha, Japan [22] Filed: Feb. 16, 1973 [21] Appl. No.: 332,792

[30] Foreign Application Priority Data Feb. 19, 1972 Japan 47-17556 [52] US. Cl. 423/239 [51] Int. Cl? B0lJ 8/00; COlB 21/00 [58] Field of Search 423/235, 239, 599, 402, 423/404, 212, 213, 395; 252/471; 55/68, 74

[56] References Cited UNITED STATES PATENTS 956,120 4/1910 Machalske 423/599 1,318,891 10/1919 MacMi1lan.... 423/599 1,616,900 2/1927 Kassner 423/395 3,207,704 9/1965 Stephens et a1 423/213 X 3,214,236 10/1965 Weisz 423/213 3,512,925 5/1970 Buechler 423/219 3,575,848 4/1971 Miale 252/471 FOREIGN PATENTS OR APPLICATIONS 226,568 9/1968 U.S,S.R... 423/239 OTHER PUBLICATIONS Petriashuili, et a1., Stability of Potassium Permanganate and Manganate in Potassium Hydroxide Solutions, Chem. Abstracts, Vol. 73, 1970, No. l15944k. Kobakhidze, Chem. Abstr., No. 8000b, Vol. 54, (1960). Purtseladze et a1., Chem. Abstr., No. 8555g, Vol. 53, (1959). Vecera et a1., Chem. Abstr.,- No. 20671a, Vol. 54, (1960).

Primary Examiner0scar R. Vertiz Assistant Examiner-Brian E. Hearn Attorney, Agent, or Firm-Wenderoth, Lind Ponack [57] ABSTRACT A method for removing oxides of nitrogen from gaseous mixtures by contacting the gaseous mixtures with alkali manganate.

3 Claims, No Drawings METHOD FOR REMOVING OXIDES NITROGEN FROM GASEOUS MIXTURES This invention relates to a method for removing oxides of nitrogenfrom gaseous mixtures containing such oxides of nitrogen. More particularly, it is concerned with a removal of the oxides of nitrogen from atmospheric air and other gaseous mixtures containing therein oxides of nitrogen such as nitrogen monoxide (NO) and nitrogen dioxide (N by chemical treatment of such gaseous mixtures with a treating agent consisting of alkali manganate which is capable of oxidizing NO and N0 and of reactively absorbing N0 It has been too well recognized that exhaust gases from internal combustion engines, nitric acid produc tion plants, thermoelectric power plants, and so forth contain considerable amount of oxides of nitrogen which is liable to bring about public harm (or the socalled air-pollution) in the form of photo-chemical smog, etc. And, measures have already been taken on a practical and industrialized scale against the nitric acid production plants, in particular, which discharge a large quantity of waste gas containing high concentration of oxides of nitrogen, so as to remove such undesirable pollutant by various ways such as washing with alkali, contact-decomposition of the pollutant with hydrogen, methane, etc., in the presence of a metallic oxide catalyst, or others. In case, however, the exhaust gas contains low concentration of oxides of nitrogen such as that discharged from thermo-electric power plant or a small scale internal combustion engine, or such exhaust gas is discharged and scattered around in a small quantity, there has so far been no effective method of removing the oxides of nitrogen from such exhaust gas, and this has resulted in great social problem.

As the method of treating exhaust gas from small scale internal combustion engines, there has been conducted researches and studies mainly focussed on the contact-decomposition of the oxides of nitrogen with a reductive substance such as hydrogen, hydrocarbons, ammonia, carbon monoxide, etc. in the presence of a catalyst principally composed of metallic oxides of iron, copper, chromium, nickel, platinum, cobalt, aluminum, molybdenum, vanadium etc., to render the exhaust gas non-poisonous. However, no satisfactory result has been obtained for the reason that activity of the catalyst rapidly lowers due to adherence thereto of tars and ashes in the exhaust gas as well as deposition of carbon substances.

On the other hand, removal of the oxides of nitrogen by reactive absorption has been publicly known, and solutions of ferrous sulfate and potassium permanganate have been known as an aqueous absorbent to NO, solutions of alkali hydroxide and alkali carbonate to N0 and PbO and Na O have also been known as a solid absorbent to NO, and alkali hydroxide, alkali carbonate, and PbO to N0 However, this absorption method has yet to attain its application in an industrialized scale to remove oxides of nitrogen in the gaseous mixtures containing therein both NO and N0 since there has been discovered no effective absorbent capable of absorbing both NO and N0 simultaneously.

As the result of reviewing and studying made by the present inventors on the aforementioned known methods of removing oxides of nitrogen, it has been con- 2 cluded that the fundamental solution to removal of the oxides of nitrogen is nothing but the reactive absorption, based on which conclusion further researches have been conducted on various sorts of compounds to determine those having ideal capability of absorbing the oxides of nitrogen.

The following view points have been established as the result of the researches.

1. NO, in its original form, can be absorbed in only a very special type of absorbent such as ferrous sulfate, so that kind of the absorbent is extremely limited.

2. N0 is readily absorbed in alkali metal compounds of alkaline property.

3. Accordingly, in the absorption of the oxides of nitrogen, the oxidation process of NO constitutes an important factor, for the perfect oxidation of which any one of the following ways should be resorted to.

a. NO is first oxidized to NO by use of a strong oxidizer, followed by absorption of N0 by alkaline substance.

b. By use of an absorbent having both oxidizing and alkaline properties, the nitrogen oxide is absorbed at once.

Based on this notion, detailed examinations were made on various sorts of compounds having practical absorbing capability, least toxicity, least inflammability, least explosiveness, and being available at a low price, and have finally found out that alkali manganate having both oxidizing and alkaline properties, and a substance obtained by heat-treating manganese ore with alkali hydroxide possess far superiority to the abovementioned PbO in its absorbing capability to the oxides of nitrogen, and yet are not instable as Na O It is therefore a primary object of the present invention to provide an effective method of removing oxides of nitrogen (NO, N0 present in exhaust gas from various sources.

It is another object of the present invention to provide a method of removing oxides of nitrogen (NO, N0 existing in gaseous mixtures by subjecting the gaseous mixture to contact-reaction with a treating agent containing alkali manganate, or another treating agent containing both alkali compound and a manganese-containing substance which produces alkali manganate at the time of using, thereby reactively absorbing NO and N0 into this agent. i

The foregoing objects and actual reactions to take place during the treatment will become more clearly understood from the following detailed description of the invention in conjunction with a few preferred examples.

For alkali manganate, there have been known potassium manganate (K MnO and sodium manganate (Na MnO both of which are not available in general market. However, they can be produced fairly easily. For example, potassium manganate is produced as an intermediate product, when KMnO is to produced, by adding potassium hydroxide to manganese ore and heat-treating the same at a temperature of 250 to 300C. The reaction in this case is as follows.

MnO ZKOH /O K MnO 4- H2O For the production of KMnO this intermediate product from the heat reaction is eluted with water to prepare an aqueous solution of K MnO which is further electrolytically oxidized to KMnO Thus, K MnO is the intermediate product to be obtained in the course of KMnO production, so that it can be obtained in less process steps than to obtain KMnO and at a low cost. Na MnO can also be produced in the same manner as K MnO Both K MnO and Na MnO are not necessarily pure. Rather, those which have been obtained by heat-treat- 4 from various sources such as small scale internal combustion engines, nitric acid production plants, thermoelectric power plants, and so forth.

As stated in the foregoing, the present invention,

ing manganese ore or manganese ocean nodules to- 5 which utilizes a particular absorbing agent principally gether with alkali hydroxide at a temperature to proconsisting of alkali manganate, can exhibit highly imduce alkali manganate possess excellent absorbing caproved rate of removal of oxides of nitrogen in comparpability to oxides of nitrogen without purification. Furison with the conventional methods, hence it provides thermore, when such alkali manganate is to be used at extremely effective means to solve the air-pollution a temperature of 200C and above, a simple mixture of problems we are facing nowadays, the industrial merit manganese ore and alkali hydroxide can even exhibit of which is therefore considered great. such excellent absorbing capability without it being For the purpose of enabling those skilled in the art to subjected to any preliminary heat-treatmemt to turn reduce this invention into practice, the following prethe same into alkali manganate. Accordingly, there is ferred examples are presented. It should, however, be no stringent requirement for purity of alkali manganate noted that these examples are illustrative only and do us ctll for the purposes of the present invention. not limit the scope of the present invention.

e method according to the resent invention can be applied to both wet and dry eatments. The treat- EXAMPLE I ment temperature for the dry treatment ranges from Quartz boards, on which various kinds of sample 200 to 600C, or, more preferably, from 300 to absorbing agents in powder in aquantity of 0.3 to 0.7 g 500C. Since alkali manganate does not undergo emiare placed, are accommodated in a quartz reaction nent decomposition upto 600C, it can be safely used in tube maintained at a temperature of 400C in an electhe dry treatment. Moreover, alkali manganate, when it tric furnace. is dissolved in water, is hydrolyzed to become KMnO Into this quartz reaction tube, a gaseous mixture and deposits manganese oxide (MnO As it does not containing 6.4% or so of oxides of nitrogen (NO,), in hydrolyze in alkali hydroxide solution, but dissolves in which the ratio of NO to N0 is l 10, is introduced, it, the compound is usually used in alkaline solution in and subjected to reaction with the absorbents for 2 case of the wet treatment. hours.

The reactions between potassium manganate After 2 hours reaction, the absorbing capability of (K MnO and the oxides of nitrogen (NO, N0 are as each absorbent is measured, and the results as shown in shown in the following chemical equations, the following Table 1 is obtained. (The abovemen- 1 3 8; tioned blo -containing gas is prepared by burning air 3K2Mn0 2N0 ZHZO 2km)a 4K0 containing about 6 of ammonia at approximately 3 n02 800C while passing through a platinum catalyst, there- The absorbing capability of alkali manganate with ft li i i h water, d removing moisture respect to the oxides of nitrogen is invariable even in th f h h a dessicator containing phosphorus P Presence of carbon monoxide carbon diOX- pentoxide. The absorbed quantity of the oxides of inlde 2)- trogen is measured by the Kjeldahl method, and ex- The method of the present invention is applicable to pressed in terms f thequantity f 0 remove the oxides of nitrogen existing in exhaust gas 40 AS i apparent from Table 1 h b bi agent Table 1 Absorbing Capability of NO of Absorbing Agent at 400C (1) 300 g of primary KOH and g of primary KMnO. are dissolved in 500 ml of water. and the solution was boiled for 7 hours. After the boiling, it was cooled, and the produced crystals of K MnO were filtered and dessicated with silica gel. 4 (2) To pulverized manganese ocean nodule ofmeshes (Mn content of 23.3%), KOH is added so as to make the KOH/MnO, mol/l ratio to be 2.5. and the mixture is roasted for 5 hours at 450C.

(3) To pulverized soft manganese or of-150 meshes (Mn content of 48.5%), NaOH is added so as to make the NaOHIMnO, mol/l ratio to be 2.5. and the mixture is roasted for 2 hours at 350C.

(4) A mixture of 0.3186 g of K,CO, and 0.3458 g of MnO,

(5) A mixture of 0.3598 g of NaOH and 0.3072 g of MnO,

(6) Primary reagent is used as it is.

provided with conditions which produce or contain potassium manganate or sodium manganate possesses far greater absorbing capability of NO than lead oxide and potassium permanganate which are known to be the conventional absorbent of NO, and to have the absorbing action only in the state of aqueous solution.

EXAMPLE 2 1n the same manner as in Example 1 above, variations in absorbing quantity of potassium manganate with respect to NO, depending on temperature are examined, and the results shown in the following Table 2 is obtained.

Table 2 Change in Absorbing Quantity of NO, by K MnO due to Temperature Variations 6 EXAMPLE 3 Two absorbing bottles (30 mm 250 mm) are connected in series, into which a mixed solution of potassium manganate (0.2 mol) and caustic potash (2 mols) is put in a quantity of 60 ml and 40 ml, respectively, and MO -containing gas as used in Example 1 is introduced at a rate of 500 ml/min. for 2 hours. The rate of absorbing NO, reached 100%.

In the same experiment using an aqueous solution of potassium permanganate, the ratio of absorption of NO, is as low as in comparison with the case of using aqueous solution of potassium manganate.

What we claim is:

l. A method for removing oxides of nitrogen (N0 existing in gaseous mixtures which consists essentially of subjecting said gaseous mixtures to contact-reaction with an alkali metal manganate at a temperature of from 200C to 600C, thereby absorbing the oxides of nitrogen into said alkali metalmanganate.

2. The method according to claim 1, in which said alkali metal manganate is one selected from the group consisting of potassium manganate and sodium manganate.

3. The method according to claim 1, in which the contact reaction is conducted at a temperature range of 300 to 500C. 

1. A METHOD FOR REMOVING OXIDES OF NITROGEN (NOX) EXISTING IN GASEOUS MIXTURES WHICH CONSISTS ESSENTIALLY OF SUBJECTING SAID GASEOUS MIXTURES TO CONTACT-REACTION WITH AN ALKALI METAL MANGANATE AT A TEMPERATURE OF FROM 200*C TO 600*C, THEREBY ABSORBING THE OXIDES OF NITROGEN INTO SAID ALKALI METAL MANGANATE.
 2. The method according to claim 1, in which said alkali metal manganate is one selected from the group consisting of potassium manganate and sodium manganate.
 3. The method according to claim 1, in which the contact reaction is conducted at a temperature range of 300* to 500*C. 